The present invention relates generally to the field of polishing pads for chemical mechanical polishing. In particular, the present invention relates to a method for forming grooves in the surface of a polishing pad.
Integrated circuit manufacturing consists of a series of layering steps in which conductive, semiconductive or dielectric materials are deposited onto, or removed from, the surface of a substrate such as a silicon wafer. These layers are deposited by techniques such as sputtering, chemical vapor deposition or electrochemical plating. These layers may also be etched to create channels or holes into which subsequent layers may be deposited.
The deposition process, especially when used to fill channels or holes, results in a non-planar surface. Before subsequent layers of an electronic device can be deposited, the surface of the wafer must be planarized. Chemical mechanical polishing or planarization (CMP) is frequently used to prepare the surface of the wafer for further processing by removing excess material deposited in the layering process or exposing underlying material. The CMP process both removes large scale topography, such as artifacts left by channels or holes after they have been filled by a deposition process, and eliminates small scale imperfections, such as scratches.
In a typical CMP process a semiconductor substrate is mounted on a rotating carrier assembly and contacted with a polishing pad in a CMP apparatus. The carrier assembly provides controlled force, pressing the substrate against the polishing pad and optionally provides lateral motion in addition to rotation. The polishing pad is mounted on a rotating platen; and a polishing composition is supplied to the surface of the polishing pad such that the polishing composition flows over the surface of the polishing pad and enters the gap between the polishing pad and the substrate. The polishing composition contains appropriate chemistry for polishing the substrate and optionally contains abrasive particles to aid in the removal of substrate material. The mechanical action of the polishing surface and the abrasive particles against the substrate surface acts in conjunction with chemical interaction between the substrate and the polishing composition to planarize the surface of the substrate, which is then prepared to receive another layer by a deposition process or can otherwise be altered, as through etching.
In order to facilitate the transportation and delivery of the polishing composition to the surface of the substrate, and thereby facilitate the CMP process, it is often desirable to create asperities in the polishing surface of the polishing pad. Asperities may be created by embedding particles in the polishing pad during manufacture, such as soluble particles or hollow capsules. The soluble particles dissolve when exposed to the polishing composition, leaving behind an asperity. The embedded capsules may be ruptured, thereby exposing the hollow core to the polishing surface, creating an asperity. In either case, polishing composition fills the asperity and is transported by the motion of the polishing pad, relative to the substrate, and delivered to the surface of the substrate. Asperities also serve to collect used polishing composition, polishing composition that has reacted with the surface of the substrate and debris, caused by the mechanical action, from the gap between the polishing surface and the substrate. As the CMP process continues, the polishing surface is worn away, exposing new particles or capsules to the polishing surface, thereby regenerating the asperities.
Another method for creating and regenerating asperities on the surface of the polishing pad is through conditioning. Typically, conditioning involves abrading or cutting the polishing surface with a plurality of diamond points or other mechanical means. The diamond points are typically embedded in a conditioning pad that is pressed against the polishing surface while the polishing pad rotates, and is optionally rotated, oscillated or otherwise moved relative to the polishing pad. The diamond points create small holes or channels that fill with and transport the polishing composition and remove debris material. The channels created by conditioning may also serve to aid in the flow of the polishing composition over the surface of the polishing pad as discussed below.
An important consideration in the design of a CMP process is the rate of flow of the polishing composition over the polishing surface. In order to control the flow of the polishing composition, grooves may be added to the polishing surface. Various factors such as groove dimensions, shape and orientation affect the rate of flow of the polishing composition both into and out of the gap between the substrate and the polishing surface, and the overall rate of consumption of the polishing composition. Optional groove configurations include circular, spiral, x-y, and radial. Grooves are typically created by cutting the polishing surface on a lathe after the polishing pad has been formed. Alternatively, grooves may be pressed or stamped into the polishing pad, or may be created in a molding process.
Parallel groove designs such as circular, spiral and x-y grooves may be formed by a cutting device with a plurality of fixed blades. For example, a large number of circular grooves may be cut simultaneously using a lathe with a plurality of fixed blades. Optionally a lathe can be used to form grooves in a polishing pad with a rotating bit. One of the advantages of a lathe is that the polishing pad can be rotated very quickly under a stationary blade, which allows grooves to be formed at a high rate even if only a single blade is used.
In order to form some groove configurations (typically diverging groove patterns), such as straight or curved-radial grooves, a CNC machine is used. In CNC milling machines, a rotating bit moves in the X, Y and Z directions relative to the polishing pad. The rotating bit is typically rotated at low speeds, and may optionally be tilted or moved laterally. A CNC machine can be used to create intricate patterns not achievable on a lathe, but has the disadvantage of being able to produce only one groove at a time. The use of a CNC machine therefore results in a manufacturing process that is time consuming and inefficient.
One method for forming grooves in a polishing pad is disclosed in U.S. Pat. No. 6,340,325 to Chen et al. The method involves the use of a router with a rotating bit opposing a platen on which is mounted a polishing pad. The router and the platen move with respect to one another in three axes. The router is lowered so that the bit cuts an initial hole to a desired depth not greater than the thickness of the pad, and is then moved laterally to create the desired groove pattern. According to this method, the groove pattern is formed by one continuous groove, and the groove depth may vary, but never exceed a maximum amount determined by the distance of protrusion of the router bit beyond integral stops.
Research in the area of the effect of groove configuration on a CMP process shows that certain groove configurations have beneficial effects on the CMP process, such as improving wafer uniformity, eliminating small scale topography from the polished surface of a substrate, or reducing the rate of consumption of the polishing composition. These groove configurations have become more important to the satisfactory performance of a CMP process in the future as integrated circuit manufacturing tolerances decrease and a greater degree of planarity on a smaller scale is required at less operating cost. Many of these beneficial groove configurations are not able to be made on a lathe, and require use of a CNC machine, which, as discussed above, is less efficient.
What is needed is a method of forming groove patterns in a CMP polishing pad with a lathe or a CNC machine using a rotating bit, which is more efficient and allows for a greater rate of production of grooved CMP polishing pads at an acceptable level of quality.